1. Field of the Invention
The present invention relates to a method for selectively or nonselectively etching a semiconductor substrate or layer constituted principally of silicon or silicon carbide, and more particularly to a method for etching such a semiconductor substrate or layer through the use of a reactive gas plasma.
2. Description of the Prior Art
Heretofore, there have been proposed a wide variety of semiconductor devices employing a semiconductor substrate or layer constituted principally of silicon or silicon carbide. Such semiconductor devices are usually fabricated by a manufacturing method including a step of selectively or nonselectively etching the semiconductor substrate or layer.
Methods that have been proposed for etching the semiconductor substrate or layer are a dry etching method employing a reactive gas plasma and a wet etching method employing a reactive liquid. Recently the dry etching method has been used widely because it is superior to the wet etching method in precision and in minuteness.
The conventional dry etching process usually employs, as the reactive gas plasma, a plasma of a gas containing carbon and a halogen, such as CF.sub.4, CHF.sub.3, CF.sub.3 Br or CCl.sub.4 gas, or a plasma of a gas mixture of such a gas containing carbon and a halogen and a hydrogen, nitrogen or oxygen gas.
However, in the case of etching the semiconductor substrate or layer formed principally of silicon or silicon carbide by the dry etching process using the plasma of a gas containing carbon and a halogen, the carbon, its compound with the halogen and the halogen enter into the semiconductor substrate or layer. As a result of this, the composition of the semiconductor substrate or layer undergoes variations, preventing the etching thereof. Furthermore, the characteristic of the substrate or layer is degraded, and in some cases, it is insulated, in particular, by the carbon. Sometimes recombination centers are created by virtue of the halogen (except fluorine) in the semiconductor substrate or layer. When using a plasma of the gas mixture of the gas containing carbon and a halogen and oxygen gas, the abovesaid difficulties are encountered, and at the same time, oxygen enters into the semiconductor substrate or layer to insulate it.
When using a plasma of the gas containing carbon and a halogen or a plasma of the abovesaid gas mixture incuding such a gas, the carbon is left as a residue on the surface of the semiconductor substrate or layer. This carbon residue hinders the etching of the semiconductor substrate or layer and contaminates its surface.
The abovesaid reactive gas plasma is usually created from the reactive gas by applying thereto a high-frequency power. In the case of using, as the reactive gas plasma, a plasma of the abovementioned gas containing carbon and a halogen, or the aforesaid gas mixture containing it, the etching rate of the semiconductor substrate or layer is low unless the high-frequency power applied to the reactive gas for producing its plasma is sufficiently high or unless the semiconductor substrate or layer is heated by an electric heater to sufficiently high temperatures. Accordingly, the etching inevitably consumes much power or much time.
For selectively etching the semiconductor substrate or layer into a required pattern through using the reactive gas plasma, it is customary to use a mask layer formed on the semiconductor substrate or layer. Conventionally, the mask layer is formed of silicon nitride, silicon oxide, Al, Cr, Ni, Co, Ta, W, Mo or like metal, or photoresist, in the case of using, as the reactive gas plasma, a plasma of the gas containing carbon and a halogen or a plasma of the gas mixture containing it. However, the etching rate of the mask layer is lower than the etching rate of the semiconductor substrate or layer by the abovesaid gas plasma or gas mixture plasma, but the mask layer is inevitably etched at a relatively high rate. That is to say, the ratio between the etching rate of the semiconductor substrate or layer and etching rate of the mask layer is relatively small. This leads to the defects that the mask layer must be formed thick and that the semiconductor substrate or layer cannot be etched into a required pattern with high precision.
The etching of the semiconductor substrate or layer through the use of the reactive gas plasma is usually performed by placing the semiconductor substrate or layer in a reaction chamber, forming the plasma from the reactive gas and passing the plasma over the surface of the semiconductor substrate or layer by the use of an exhaust pump connected to the reaction chamber from the outside. In this case, when the gas containing carbon and a halogen (except fluorine) is used, the exhaust pump is attacked by a halogen (except fluorine) gas and a compound gas of the carbon and the halogen (except fluorine). Therefore, the conventional etching process utilizing a plasma of the gas containing carbon and a halogen shortens the lifetime of the exhaust pump.